1. Field of the Invention
The present invention relates to a magnetic particle imaging apparatus that forms an image of a distribution of magnetic particles based on change in magnetic flux caused by magnetization of the magnetic particles, a method of disposing a detection coil for the magnetic particle imaging apparatus, and a magnetic flux detecting apparatus. In particular, the present invention relates to a technology for enhancing detection sensitivity of the detection coil.
2. Description of the Related Art
In recent years, a method has been proposed for injecting magnetic particles, such as super paramagnetic iron oxide, that serve as a contrast medium into a subject and forming an image of a distribution of the contrast medium (refer to, for example, JP-A 2003-199767 (KOKAI) or B. Gleich, J. Borgert, J. Weizenecker, “Magnetic Particle Imaging (MPI)”, Philips Medic Mundi Vol. 50 No. 1, 2006/5 [online], May 23, 2007, retrieved from the Internet: URL: http://www.medical.philips.com/main/news/assets/docs/medi camundi/mm_vol50_no1/12_Gleich.pdf). This method is called magnetic particle imaging. FIG. 16 is a diagram for explaining a principle of magnetic particle imaging. In magnetic particle imaging, for example, a static magnetic field 1 flowing in vertically opposite directions is generated using a permanent magnet in an area in which magnetic particles are distributed.
At this time, a magnetic field from above and a magnetic field from below are mutually cancelled at an approximate center of the static magnetic field 1, thereby generating an area 2 where a localized magnetic field becomes zero. The area 2 is referred to as a “field free area”. Then, a modulation coil 4 for generating a modulation magnetic field and a detection coil 5 for detecting a change in an interlinking magnetic flux are disposed within the static magnetic field 1.
Here, it is assumed that a modulation magnetic field 3 is generated in the area in which the magnetic particles are distributed by applying an electrical current to the modulation coil 4. At this time, magnetic saturation occurs in areas other than the field free area 2 because of the static magnetic field 1. Therefore, the magnetic flux within the areas other than the field free area 2 does not change even when the modulation magnetic field 3 is applied. On the other hand, magnetic saturation does not occur in the field free area 2 because the magnetic field is near to zero. When the modulation magnetic field 3 is applied, the magnetic particles present in the field free area 2 become magnetized. A magnetic flux is generated from the field free area 2 with the magnetization of the magnetic particles.
The magnetic flux generated from the field free area 2 causes a change in the magnetic flux interlinked with the detection coil 2. The change in the magnetic flux appears as a change in voltage induced in the detection coil 5. An amount of change in the voltage depends on an amount of magnetic particles present in the field free area 2. In other words, the voltage induced in the detection coil 5 changes based on the amount of magnetic particles present in the field free area 2.
When the above-described principle is used, an image of the distribution of the magnetic particles within the subject can be formed by measuring a change in the voltage induced in the detection coil being measured while the field free area is gradually moved within the subject into which the magnetic particles have been injected. In recent years, a deliberation has begun on clinical application of the above-described magnetic particle imaging.
In the above-described magnetic particle imaging, the change in the voltage induced by the magnetization of the magnetic particles is required to be measured using the detection coil. However, voltage caused by the modulation magnetic field applied by the modulation coil is also induced in the detection coil. The detection coil detects voltage as a modulation signal (such as high-frequency signal). However, when the voltage caused by the modulation magnetic field is also induced, a signal indicating the change in the voltage caused by the magnetization of the magnetic particles and a signal indicating change in the voltage caused by the modulation magnetic field are detected in an overlapping state. Therefore, when an image of the distribution of the magnetic particles is formed, it is required to extract only the signal of the voltage induced by the magnetization of the magnetic particles from the signals detected by the detection coil.
However, the voltage induced by the modulation magnetic field is significantly larger than the voltage induced by the magnetic particles. Therefore, separation of the respective signals of the voltages becomes difficult. This problem becomes more significant in clinical application.
For example, when a magnetic particle imaging apparatus scaled for humans is configured based on a description written in “Magnetic Particle Imaging (MPI)” by B. Gleich, J. Borgert, and J. Weizenecker, and it is assumed that a magnitude of the modulation magnetic field applied by the modulation coil is 10 mT/μ0, that is a strength facilitating magnetic saturation of the magnetic particles, the voltage induced in the detection coil by the modulation magnetic field is about 150 volts. On the other hand, for example, when an early stage cancer cell of 10 mm3 indicating diamagnetism having a magnetic susceptibility of −7.1×10−6 is the subject, the voltage induced in the detection coil is about 50 nanovolts.
A known document describes a separation method using frequency as a method for separating the signals. Specifically, the method takes advantage of the signal of the voltage induced by the magnetization of the magnetic particles including distortion, whereas the signal of the voltage induced by the modulation magnetic field is a sine wave. A harmonic component is extracted from the signals detected by the detection coil. As a result, only the signal of the voltage induced by the magnetization of the magnetic particles is extracted.
However, when taking into consideration of clinical application, because the voltage induced by the modulation magnetic field is significantly larger than the voltage induced by the magnetization of the magnetic particles, as described above, it id difficult to obtain sufficient detection sensitivity even using this method. Therefore, to apply the magnetic particle imaging to a clinical application, the detection sensitivity of the detection coil is required to be significantly enhanced compared to a conventional detection coil.